The extended super frame format used in the public switched telephone network (PSTN) employs 24, 8-bit symbols from which a least significant bit (LSB) is preempted (“robbed”) every 6 symbols for network signaling purposes in a scheme called robbed bit signaling (RBS). The super frame format has 4 RBS frames, each RBS frame composed of 6, 8-bit symbols. According to convention, LSBs of the first RBS frame are called the ‘A’ bits, those from the second RBS frame are known as ‘B’ bits while the signaling bits in the third and fourth RBS frames are called the ‘C’ and ‘D’ bits. Between network switches RBS is used for synchronization. Each RBS super frame consists of 4 RBS frames and each frame has 6 phases.
Within the PSTN it is known which bits have been robbed for signaling and what their signaling levels are. However, this information is not known at the client site (customer's) interface to the PSTN. If it is desired to use the PSTN to transmit data, the client site interface must synchronize to the sampling rate and levels used in the digital trunks.
In the prior art, it was the practice in the network to rob the LSB only of symbols represented by an even level PCM code. This allowed the RBS pattern to always be ABDC=1111. A client site modem could therefore be trained on samples of a level that would not be subject to robbing. For example, in U.S. Pat. No. 6,178,185, a client site modem could send a test pattern composed of high-amplitude symbols to train a receiving client site modem because the training level could be at a different level than would be used for bit robbing.
More recently, however, network signaling systems are being deployed that do not always subject the same symbol level to having its LSB robbed for network signaling purposes. In random RBS, the ABCD pattern may change during the four robbed bit frames of the extended super frame. When the signaling level pattern changes it becomes necessary for the client site's modem to learn the signaling levels during the training sequence. In Wang et al, U.S. Pat. No. 6,185,250, and in a co-pending application entitled “Improved Equalizer Training In the Presence of Network Impairment”, filed on Jun. 22, 1999, Ser. No. 09/338,664, arrays of slicer tables are employed to detect the levels used for network signaling. Briefly, a training sequence of analog symbols is transmitted to the client site's modem to enable the modem to construct a slicer table to learn what levels are being employed at the network. The substance of the aforementioned patents is incorporated herein by reference.
Unfortunately, when the training sequence passes through the PSTN, the network may rob a bit position of the training sequence, thereby corrupting its digitally encoded value. Moreover, in some networks, not only may the original signal be robbed by the ABCD pattern, but digital loss may be applied for network echo cancellation purposes. In addition, if the signal is passed over a subscriber line carrier system, the signal may go through another robbed bit signaling process. The final result is that bits may change value between the transmitter and the receiver, thereby injecting a serious noise component into the level learning procedure as well as slowing down the analog modem's construction of its slicer table. Since there are four RBS frames and 6 phases in each frame, detecting the robbed positions would seem to require an array of twenty-four slicers, each of which must attempt to ascertain the slicing level for its phase of the RBS super frame. This is represents a large commitment of hardware and software resources.